1. Field of the Invention
The present invention relates to a diode and more particularly, to a high breakdown-voltage diode having an electric-field relaxation region relaxing an electric field existing in the vicinity of a p-n junction.
2. Description of the Prior Art
In recent years, Silicon On Insulator (SOI) wafers or substrates have been widely used for fabricating high withstand-voltage semiconductor elements or components and semiconductor integrated circuit devices in which high withstand-voltage semiconductor elements or components and related circuits are mounted on a same semiconductor chip. This is because an SOI substrate increases the isolation withstand voltage of electronic elements or components implemented thereon.
FIG. 1 shows a partial cross section of a conventional high breakdown-voltage diode formed on an SOI substrate. This diode has a breakdown voltage of 150 V.
In FIG. 1, the conventional high breakdown-voltage diode 101 includes an SOI substrate 120. This substrate 120 consists of a p.sup.- -type single-crystal silicon supporting plate 102, a silicon dioxide (SiO.sub.2) layer 103 formed on the plate 102, and an n.sup.- -type single-crystal silicon layer 104 formed on the SiO.sub.2 layer 103. The p.sup.- -type supporting plate 102 has a doping concentration of 1.times.10.sup.14 atoms/cm.sup.3. The SiO.sub.2 layer 103 has a thickness of 1 .mu.m. The n.sup.- -type single-crystal silicon layer 104 has a doping concentration of 5.times.10.sup.15 atoms/cm.sup.3 and a thickness of 5 .mu.m.
An isolation region 107 made of SiO.sub.2 is formed in the n.sup.- -type silicon layer 104 to surround an area where the diode 101 is formed, defining a rectangular-shaped device region 104a in the layer 104. The device region 104a is laterally isolated by the isolation region 107 and vertically isolated by the SiO.sub.2 layer 103 from other electronic elements or components located outside the device region 104a.
A p.sup.+ -type diffusion region 105 serving as an anode region of the diode 101 is formed in the surface area of the device region 104a by selective diffusion. The surface of the p.sup.+ -type anode region 105 is exposed from the single-crystal silicon layer 104 and is apart from the isolation region 107. The anode region 105 has a doping concentration of 1.times.10.sup.19 atoms/cm.sup.3 and a depth of 1 .mu.m from the surface of the layer 104. An n.sup.+ -type diffusion region 106 serving as a cathode region of the diode 101 is formed in the surface area of the device region 104a by selective diffusion. The surface of the n.sup.+ -type cathode region 106 is exposed from the single-crystal silicon layer 104 and apart from the isolation region 107 and from the anode region 105. The cathode region 106 is formed in such a way that the distance between the opposing edges of the anode and cathode regions 105 and 106 is 15 .mu.m. The cathode region 106 has a doping concentration of 1.times.10.sup.19 atoms/cm.sup.3 and a depth of 1 .mu.m from the surface of the layer 104.
An anode electrode 108 is formed on the exposed surface of the p.sup.+ -type anode region 105. The anode electrode 108 is electrically connected to the anode region 105.
A cathode electrode 109 is formed on the exposed surface of the n.sup.+ -type cathode region 106. The cathode electrode 109 is electrically connected to the cathode region 106.
With the conventional high breakdown-voltage diode 101 shown in FIG. 1, when a reverse voltage is applied across the anode and cathode electrodes 108 and 109 on operation, an electric field is generated in the device region 104a between the p.sup.+ -type anode region 105 and the n.sup.+ -type cathode region 106. The electric field thus generated is relaxed by the part of the n.sup.- -type silicon layer 104 between the anode and cathode regions 105 and 106. The magnitude of the relaxation effect to the electric field varies dependent upon the distance between the anode and cathode regions 105 and 106.
FIG. 2 shows a partial cross section of another conventional high breakdown-voltage diode, which has the double REduced SURface Field (RESURF) structure increasing the breakdown voltage.
In FIG. 2, the conventional high breakdown-voltage diode 201 includes a p.sup.- -type single-crystal silicon substrate 202. An n.sup.- -type well 203 is formed in the substrate 202 by selective diffusion.
A p.sup.+ -type diffusion region 204 serving as an anode region of the diode 201 is formed in the surface area of the substrate 202 to be overlapped with the boundary of the well 203 by selective diffusion. The surface of the p.sup.+ -type anode region 204 is exposed from the substrate 202.
An n.sup.+ -type diffusion region 205 serving as a cathode region of the diode 201 is formed in the surface area of the substrate 202 by selective diffusion. The cathode region 205 is not overlapped with the boundary of the well 203 and is apart from the anode region 204. The surface of the n.sup.+ -type cathode region 205 is exposed from the substrate 202.
A p.sup.- -type electric-field relaxation region 206 is formed in the surface area of the substrate 202 between the anode and cathode regions 204 and 205 by selective diffusion. The surface of the p.sup.- -type electric-field relaxation region 206 is exposed from the substrate 202. The relaxation region 206 is apart from the anode and cathode regions 204 and 205.
An anode electrode 207 is formed on the exposed surface of the p.sup.+ -type anode region 204. The anode electrode 207 is electrically connected to the p.sup.+ -type anode region 204.
A cathode electrode 208 is formed on the exposed surface of the n.sup.+ -type cathode region 205. The cathode electrode 208 is electrically connected to the n.sup.+ -type cathode region 205.
An electric-field relaxation electrode 209 is formed on the exposed surface of the p.sup.- -type electric-field relaxation region 206. The electric-field relaxation electrode 209 is electrically connected to the electric-field relaxation region 206.
Although not shown in FIG. 2, the anode electrode 207 is electrically connected to the electric-field relaxation electrode 209, so that these two electrodes 207 and 209 have the same electric potential on operation.
With the conventional high breakdown-voltage diode 201 shown in FIG. 2, a reverse voltage is applied across the anode and cathode electrodes 207 and 208 on operation. At the same time, the same reverse voltage is applied across the electric-field relaxation electrode 209 and the cathode electrode 208, because the anode and electric-field relaxation electrodes 207 and 209 are electrically connected to one another.
Due to the applied reverse voltage, a first depletion region (not shown in FIG. 2), which is formed at the p.sup.- n.sup.- junction of the p.sup.- -type electric-field relaxation region 206 and the n.sup.- -type well 203, grows upward and downward. As a result, the upper end of the first depletion region reaches the surface of the well 203, thereby depleting the relaxation region 206 completely. The lower end of the first depletion region approaches the interface between the n.sup.- -type well 203 and the p.sup.- -type remaining substrate 202
On the other hand, a second depletion region (not shown in FIG. 2), which is formed at the p.sup.- -n.sup.- junction of the n.sup.- -type well 203 and the p.sup.- -type remaining substrate 202, grows upward and downward due to the applied reverse voltage. The upper end of the second depletion region reaches the lower end of the first depletion region in the well 203, resulting in merge of the first and second depletion regions. Thus, the part of the well 203 below the p.sup.- -type electric-field relaxation region 206 is completely depleted.
As described above, the electric field existing in the vicinity of the p.sup.+ -n.sup.- junction of the n.sup.- -type well 203 and the p.sup.+ -type anode region 204 is relaxed due to the existence of the first and second depletion regions. In other words, a large part of the applied reverse voltage acts on the first and second depletion regions. As a result, the effective voltage to form the electric field in the vicinity of the p.sup.+ -n.sup.- junction is decreased, increasing the breakdown voltage of the conventional diode 201.
However, the above conventional high breakdown-voltage diodes 101 and 201 have the following problems.
With the conventional diode 101 shown in FIG. 1, the electric field generated in the device region 104a is relaxed by the part of the n.sup.- -type single-crystal silicon layer 104 which is vertically and laterally isolated by the SiO.sub.2 layer 103 and the isolation region 107. Therefore, the doping concentration of the n.sup.- -type single-crystal silicon layer 104 needs to be set as a comparatively low value so as to increase the width of a depletion region generated in the layer 104.
Also, to prevent the punch through phenomenon from occurring even when the depletion region formed at the p.sup.+ -n.sup.- junction of the p.sup.+ -type anode region 105 and the n.sup.- -type device region 104a laterally grows toward the cathode region 106 due to the applied reverse voltage, the anode and cathode regions 105 and 106 are required to be apart from each other at a specific distance corresponding to a wanted breakdown voltage.
Accordingly, there is a problem that the electric resistance between the anode and cathode regions 105 and 106 after breakdown occurs is high and that the diode 101 occupies a large chip area.
With the conventional diode 201 shown in FIG. 2, because the anode electrode 207 is electrically connected to the electric-field relaxation region 206, the electric potentials of the anode electrode 207 and the electric-field relaxation region 206 are always equal. Usually, these two electric potentials are set as zero V.
As a result, there is a problem that the electric potential of the anode electrode 207 is unable to be set at an optional value other than zero V on operation.